1. Field of the Invention
This invention relates to H.sub.3 PO.sub.4 /CrO.sub.3 coating baths for metal surfaces, and in particular to a method for extending the useful life of known H.sub.3 PO.sub.4 /CrO.sub.3 coating baths and to a method of applying chromium phosphate coatings.
2. Statement of the Related Art
In order to deposit high-weight chromium phosphate coatings on metal surfaces (e.g., more than about 300 mg/ft.sup.2 or about 3.24 g/m.sup.2) active coating baths are employed to treat the substrate, causing high levels of displaced metal ions to build up rapidly in the bath. Since the presence of these ions in excess results in loose, powdery coatings, the baths must be discarded and renewed at frequent intervals, which is expensive and also creates waste disposal problems. A particular problem is presented by zinc-bonded aluminum surfaces of the type prepared by processes such as the ALFUSE process, (trademark of Modine Mfg. Corp., Racine, Wisc., U.S.A.) in which high zinc deposition ratios are employed. The use of an active H.sub.3 PO.sub.4 /CrO.sub.3 coating bath on these substrates results in high levels of dissolved Zn and Al in the bath, which interfere with the coating process and rapidly decrease the useful life of the bath. Although replenishers for renewing H.sub.3 PO.sub.4 /CrO.sub.3 baths are commercially available, such prior art replenishers characteristically have CrO.sub.3 and H.sub.3 PO.sub.4 ratios comparable to fresh bath ratios; as a result, the useful life of baths replenished with these materials is not usually remarkably extended.